Method of separating magnesium from wet-process phosphoric acid

ABSTRACT

A method of separating magnesium from wet-process phosphoric acid characterized in that the magnesium is separated as a readily filterable fluoride-phosphate complex compound containing magnesium and aluminum. The fluoride ion required for formation of the magnesium-containing precipitate is preferably formed by hydrolysis of fluorine compounds normally in the crude acid. In the case of insufficient quantities of any of said precipitation components, additions of such components are made to the acid or to the reaction components from which the acid is produced. The wet-process phosphoric acid low in magnesium is especially useful for production of stable ammonium polyphosphate fertilizer solutions.

United States Patent Moore et al.

[ Feb. 15, 1972 [72] Inventors: William P. Moore, Chester; Rob R. Mac- Gregor, Prince George; William C. Sierichs, Hopewell, all of Va.

[73] Assignee: Allied Chemical Corporation, New York,

[22] Filed: Oct. 15, 1969 [21] Appl. No.: 866,752

[52] U.S.Cl... ..23/l65,71/40 [51] Int. Cl. ..C0lb 25/22 [58] FieldoiSearch ..23/165, 165 B, 165 D;71/40 [56] References Cited UNITED STATES PATENTS 3,273,713 9/1966 Parrish ..23/153 3,379,501 4/1968 Treitler et al.. .....23/165 3,442,609 5/1969 Carothers et al. .....23/165 3,481,700 12/1969 Legal et a1. ..23/165 3,206,282 9/1965 Crawford et a1. ..23/ 165 FOREIGN PATENTS OR APPLICATIONS 467,843 6/1937 Great Britain ..23/l65 1,024,924 4/1966 Great Britain ..23/l65 Primary Examiner-Oscar R. Vertiz Assistant Examiner-Gregory A. Heller Attorney-Patrick L. Henry and Fred L. Kelley [57] ABSTRACT A method of separating magnesium from wet-process phosphoric acid characterized in that the magnesium is separated as a readily filterable fluoride-phosphate complex compound containing magnesium and aluminum. The fluoride ion required for formation of the magnesium-containing precipitate is preferably formed by hydrolysis of fluorine cornpounds normally in the crude acid. In the case of insufficient quantities of any of said precipitation components, additions of such components are made to the acid or to the reaction components from which the acid is produced. The wet process phosphoric acid low in magnesium is especially useful for production of stable ammonium polyphosphate fertilizer solutions.

8 Claims, No Drawings METHOD OF SEPARATING MAGNESIUM F ROM WET- PROCESS PHOSPHORIC ACID BACKGROUND OF THE INVENTION This invention relates to the treatment of phosphoric acid and more particularly refers to an improved process for the removal of magnesium from wet-process phosphoric acid.

Phosphoric acid by the so-called wet-process is conven tionally produced by digesting phosphate rock with sulfuric acid which reacts to form phosphoric acid of a concentration of about 30 percent P by weight, and gypsum which is insoluble in the reaction products. The gypsum is separated from the phosphoric acid by the usual means suchas filtration or settling or both and the resultant weak acid may be used as an article of commerce. For many important commercial uses, such as for example production of liquid fertilizers, the weak acid is concentrated by evaporation to provide phosphoric acid containing about 50-70 percent P 0 While the wet-process for phosphoric acid is simple in con cept, it is fraught with many technical difficulties, and the resultant phosphoric acid is a highly impure m'aterial con'taining relatively large amounts of dissolved sulfates and smaller amounts of fluorides, fluosilicates and other salts of aluminum, magnesium, iron and other metals. These impurities precipitate and settle out as solids'or sludge at a slow rate occurring over an interval of many days and even months. The amount of solids which settle out varies with the purity of the phosphate rock from which the acid is derived and is generally from 0.5 to percent by weight of acid.

It is known that iron and aluminum ions present as predominant metal impurities in wet-process phosphoric acid, form gelatinous precipitates which render ammonium salt fertilizer solutions prepared therefrom thixotropic and gelatinous. U.S. Pat. No. 3,044,851, discloses that the formation ofgelatinous iron and aluminum precipitates can be prevented by "heating the acid to expel the fluoride impurities, and thereafter forming acyclic polyphosphoric acids, including pyrophosphoric acid, in the acid. Moreover, this,patent'disclosesthatmagnesium requires a far higher concentration of-polyphosphates to prevent precipitation than the same amount of other metal impurities and suggests that magnesium can be selectively removed from the dilute wet-process acid by passing 'the acid over a cation exchange resin. Unfortunately, use of a cation exchange resin is relatively expensive and time consuming.

It has also been suggested to removethe magnesium impurity by overammoniation of superphosphoric acid, which acid may be defined as phosphoric acid analyzing at least 66 percent P O The superphosphoric acid is diluted with waterand then ammoniated to a N/P O ratio of about 0.4 to'precipitate the magnesium as a complex ammonium phosphate. This procedure has the disadvantage that substantial loss of.

phosphate occurs because only a smallproportion of the complex ammonium phosphate precipitate is magnesium.

Ponding procedures have also been used to separate magnesium from wet-process phosphoric acid. Of course, such procedures are inefficient and expensive, entailingcapital :investment of large storage tanks and the maintenance-of large inventories.

SUMMARY'OF THEINVENTION An object of the present invention is'to provide an-efficient, economical method of rapidly removing soluble magnesium from wet-process phosphoric acid. Another object of the invention is to reduce the magnesium content of the 'acid without recourse to neutralization with ammonia. A further object of the present invention is toprovide a purified wetprocess phosphoric acid by removal of magnesium from the acid.

It has now been discovered that it is possible to precipitate magnesium from wet-process phosphoric acid in the form of well developed crystals which can easily be'separated'fromthe solution by settling or filtration. The precipitate consists of a complex magnesium-aluminum fluoride-phosphate comof the aluminum differs,

pound which on analysis is found to have a MgO:Al O :F weight ratio of about l:l.4:2.2. In case of insufficient quantities of the precipitation components in the crude acid, additions of such components are made to the acid solution or to the reaction components from which the acid solution is produced. For maximum precipitation of magnesium, excess aluminum and fluoride must be added to the acid.

It is essential that the fluoride for formation of the magnesium complex be present in the phosphoric acid solution as free" hydrogen fluoride or fluoride ion. Fluorine is normally present in the acid in the form of SiF, or H SiF which compounds are not effective to produce the desired magnesium complex. One embodiment of the invention involves hydrolysis of H SiF, in the acid and removal of SiF to afford hydrogen fluoride for use in the process.

DESCRIPTION OF THE INVENTION "ously stated, such acid is produced by digesting phosphate rock with sulfuric acid to form phosphoric acid and gypsum which is separated. The resultant weak acid contains dissolved fluorine, magnesium,:silicon, calcium, iron, aluminum, sulfate, potassium and other impurities.

The process involves the following steps:

a. evaporating the weak phosphoric acid at a temperature of 100 C. at a pressure below atmospheric to a concentration of 45-53 weight percent P 0 preferably 47-5] weight percent P 0 whereby the H SiF content of the acid is reduced and the hydrogen fluoride content is increased;

b. maintaining the hydrogen fluoride content of the concentrated phosphoric acid at a F/MgO weight ratio ofat least 2.2, preferably between 3 and 12;

c. maintaining the soluble aluminum content of the concentrated phosphoric acid, measured as AI- O at an Al O /M g0 weight ratio of at least 1.4, preferably between about 3 and 12;

d. maintaining the concentrated phosphoric acid at 50-l00 C. for 15-40 hours to form a precipitate comprising a crystalline fllterable magnesium-aluminum-fluoridephosphate complex compound; and separating the precipitate from the purified concentrated phosphoric acid.

Preferably, the precipitation of the magnesium-containing complex is facilitated by seeding the phosphoric acid with about l-4 weight percent precipitate from a previous operation of the process.

It is generally assumed that the fluorine in the raw phosphate rock is present as calcium fluoride and forms with tricalcium phosphate a complex compound. The aluminum is present in the raw phosphate rock, as a rule, in the form of alkali aluminum silicate. The quantity and composition of this silicate in different kinds of phosphate rock varies considerably. The fluorine is generally easily dissolved on treatment .of the phosphate rock with sulfuric acid. The solubility however, in different kinds of phosphates. The solubility rises with higher alkali content of the alkali valuminum silicate.'lt is also greater with phosphates which have been calcined at temperatures over 700 C.

.As=has already been mentioned, the fluorine is mainly dissolved in the crude phosphoric acid as silicon compounds which do not form the complex magnesium-aluminumfluoride-phosphatecompound of the present inventior'i. How ever, aportion or all of the fluoride ion required for foi-mation ofthe complex magnesium-containing precipitate can be obtained by decomposition of the'H SiF in the phosphoric acid 'inaccordancewith'the following equation:

It hasbeen demonstrated thatduring evaporation of weak wet-process phosphoric acid SiF is vaporized preferentially over hydrogen fluoride and that maximum hydrogen fluoride is present in the acid when evaporation is advanced to 45-53 percent P concentration. At higher concentrations the hydrogen fluoride is rapidly volatilized.

Table I shows F and SiO analyses and calculated H SiF and HF contents of a typical wet process phosphoric acid during evaporation from a P 0 content of 35.5 percent to a P 0 content of54 percent.

The phosphoric acid is normally evaporated at about 85l00 C. under reduced pressure to a P 0 concentration of about 45-53 weight percent, whereby the F/Si0 weight ratio of the acid is increased to between 10 and 20. Preferably, the SiO content of the acid is reduced to less than 0.2 weight percent, desirably to less than 0.1 weight percent.

Hydrogen fluoride or a soluble fluoride may be added to the concentrated phosphoric acid to obtain more complete and rapid precipitation of the magnesium complex. Preferably, the hydrogen fluoride content of the acid is maintained at a F/MgO weight ratio between about 3 and 12 during the precipitation step. Precipitation of the magnesium complex is increasingly rapid and complete withincreasing F/MgO ratio; however, a weight ratio greater than 12 gives little added benefit and leads to excessive fluoride in the product acid.

As previously discussed, aluminum compounds are usually present in wet-process phosphoric acid. In some acids, sufficient aluminum is present to form the complex magnesium compound of the present invention. However, additional aluminum may be added as an aluminum compound soluble in the reaction mixture, for example as aluminum nitrate or aluminum sulfate. Preferably, the aluminum content of the acid is maintained at an A l O /MgO weight ratio between about 3 and 12 during the precipitation step. Precipitation of magnesium is increasingly rapid and complete at Al O /MgO weight ratios greater than 3 but ratios above 12 give little added benefit.

The process evaporation temperatures of 85100 C. are not very critical. In general, lower evaporation temperatures require lower pressures which increase costs of operation, whereas higher temperatures lead to increased corrosion problems. Magnesium complex precipitation temperatures of 50-l00bL C. are also not very critical, but these temperatures appear to promote precipitation and are preferred. Filtration of the acid is very rapid when the acid is maintained at 50-l00bL C.

Phosphoric acid purified by the instant process has the magnesium content greatly reduced with little reduction in the iron values. Table II shows the extent of removal of magnesium, aluminum and iron values.

TABLE II Extent of Removal of Fe, Mg and AI Concentrated Filtered Component Acid Acid 130, 49.1 49.4 MgO 0.4 0.2 Ago, 1.2 0.9 mo, 2.: 2.0

The purified phosphoric acid of the instant invention is especially useful for production of stable ammonium polyphosphate fertilizer solutions. Typical fertilizer solutions are obtained by evaporating the purified acid to form superphosphoric acid, which is then diluted with water and ammoniated to pH 5-8. An aqueous ammoniated superphosphoric acid derived from purified wet-process phosphoric of the instant invention and having the following properties has been found to be stable for many months:

1071 by weight 34% by weight 2 to 391 by weight ca 2.0% by weight 'ca 0.1% by weight ca 0.l7r by weight Nitrogen Phosphorus (as P 0 Trace minerals iron (F0 0 calcium (CaO) magnesium (MgO) ph 6.0 Specific gravity at 60 F. 1.4 Salting out temperature 0' F.

Distribution as percent by weight ofthe ammonium phosphates present was about:

37% ammonium orthophosphate 49% ammonium pyrophosphate 8% ammonium tripolyphosphate 5% ammonium tetrapolyphosphate 1% higher ammonium phosphates.

. In the following. examples all percents are by weight unless otherwise stated.

EXAMPLE I The feed acid in this example was a typical weak wetprocess phosphoric acid produced by the sulfuric acid acidulation of phosphate rock. The feed acid analyzed 35.3% P 0 2.0% F, 1.1% SiO 0.3% MgO, 0.8% A1 0,, and 1.5% Fe O The weak wet-process phosphoric acid was concentrated to 50 percent P 0 content by evaporation at C. and 66 mm. Hg, whereby about 95 percent of the silicon was removed. Evaporation was stopped when the F /Si0 ratio had increased to 20. The concentrated acid analyzed 50.0% P 0 1.2% fluoride (about 1.1% as hydrogen fluoride), 0.4% MgO, 1.2% A1 0 0.06% SiO and 2.1% Fe O The concentrated acid was stored at about 55 C. for 40 hours to promote crystal growth and settling of the complex magnesium-alt:minum-fluoride-phosphate compound. The resulting partially clarified, heat-treated wet-process acid was then filtered.

A filtration test was conducted on the heat-treated wetprocess phosphoric acid using a 97-square-foot vertical leaftype filter. Prior to filtration, the filter leaves were precoated with 11 pounds of a diatomac eous earth precoat. The filter cloth was a close weave polypropylene. The filtration pressure was maintained constant at 45 p.s.i.g. and the temperature was 77 C. The acid fed to the filter contained 1.4 weight percent solids, 50 weight percent P 0 1.2 weight percent fluoride and 0.4 percent weight percent MgO. After 1 hour filtration, there had been collected 326 gallons of clear filtrate which analyzed 50 weight percent P 0 and 0.7 weight percent F and 0.20 weight percent MgO. P 0 recovery in the clear filtrate was 98 percent while MgO removal from the acid was 50 percent. The recovery of F6 0 in the filtrate was 93 percent, which demonstrates selective removal of MgO. The filter cake on a dry basis analyzed 11.5% P 0 14.5% MgO, 20.7% A1 0,, and 31.8% F corresponding to a MgO:Al O :F weight ratio of 111.422.

EXAMPLE ll This example demonstrates the beneficial effect on magnesium removal of adding hydrogen fluoride and aluminum sulfate to the concentrated acid.

The feed acid in this example was a wet-process phosphoric acid which analyzed 28.1% P 0.5 l% MgO, 0.88% F, 0.3l% M 0 and 0.53% SiO This weak acid was concentrated at 95 C. to a P 0 content of 48.6 percent and a SiO content of 0.16 percent. Prior to concentration, about 2 percent magnesium-containing precipitate from Example I was added to the acid solution to act as seed to facilitate precipitation. After concentration, the fluoride content was adjusted to 3.14 percent by addition of hydrogen fluoride, and the aluminum concentration, measured as M 0 was adjusted to 1.77 percent by addition of aluminum sulfate.

The concentrated acid was then stored at 67 C. for 24 hours to promotecrystal growth and settling of the complex magnesium-aluminum-fluoride-phosphate compound. The resulting partially clarified acid was filtered to give a low-magnesium phosphoric acid which analyzed 48.7% P 0 0.03% MgO, 0.76% F, 0.73% A1 0 and 0.23% SiO EXAMPLE III Retention l'iine Before Filtration. Hours MgO Content of Filtrate, Weight "/1 EXAMPLE IV The purified phosphoric acid from Example I was fed to the bottom of a subnierged-combustiomtype evaporator where it was brought in contact with a downwardly directed stream of air and combustion products having a temperature of about 700 C. On contacting the hot gases, the acid was heated to about 290 C., entrained in the gases, and dehydrated to a viscous superphosphoric acid of about 74% P 0 content, of which more than 60 percent was in the polyphosphate form. From the evaporator, the entrained acid, air and gases were passed to an entrainment separator where the acid was separated. The acid was then cooled to below 120 C.

The cooledsuperphosphoric acid was mixed with water and ammonia at controlled ratios to give an aqueous ammonium phosphate fertilizer solution analyzing 1 1% N and 34-37% P 0 The ammonium phosphate solution was chilled to about 30 C. and stored. It remained stable without precipitation of solids for many'months. This stability was attributed to the low magnesium content, in as much as similar products containing about twice as much magnesium were stable for only a few weeks.

We claim:

l. A process for separating magnesium from weak wetprocess phosphoric acid obtained by treatment of phosphate rock with sulfuric acid and containing dissolved magnesium, silicon, calcium, iron, aluminum, sulfate, fluoride and other impurities which comprises:

a. evaporating the phosphoric acid until the silicon content of the acid, measured as Si():, is reduced to less than 0.2 percent, based on the weight oithc concentrated acid;

b. maintaining the hydrogen fluoride content of the concentrated phosphoric acid at a F/MgO weight ratio olatleast maintaining the soluble aluminum content of the concentrated phosphoric acid, measured as M 0 at an Ah y/M gO weight ratio ofat least 1.4; maintaining the concentrated phosphoric acid at 50-l00 C. for at least 15 hours to form a precipitate comprising a magnesium-aluminum-fluoride-phosphate complex compound; and

e. separating the precipitate from the purified concentrated phosphoric acid.

2. A process for separating magnesium from 25-35- percent wet-process phosphoric acid obtained by treatment of phosphate rock with sulfuric acid and containing dissolved magnesium, silicon, calcium, iron, aluminum, sulfate, fluoride and other impurities, which comprises:

a. evaporating the phosphoric acid at a temperature of l00 C. at a pressure below atmospheric to a concentration of 45-53 weight percent P 0 and until the silicon content of the acid, measured as SiO is reduced to less than 0.2 percent, based on the weight ofthc concentrated acid, and the hydrogen fluoride content is increased;

b. maintaining the hydrogen fluoride content of the concentrated phosphoric acid at a F/MgO weight ratio ofat least 2.2;

c. maintaining the soluble aluminum content of the concentrated phosphoric acid, measured as AI OjQHI an Al O /M g0 weight ratio of at least 1.4;

d. maintaining the concentrated phosphoric acid at 50l00' C. for 15-40 hours to form a precipitate comprising a magnesium-aluminum-fluoride-phosphate complex compound; and i e. separating the precipitate from the purified concentrated phosphoric acid.

3. The process as claimed in claim 2 wherein the F/MgO weight ratio in step (b) is maintained between about 3 and 12 and the Al O /MgO weight ratio in step (c) is maintained between about 3 and 12.

4. The process as claimed in claim 3 wherein the phosphoric acid in step (a) is seeded with l-4 weight percent solid magnesium-aluminum-fluoride-phosphate complex having a MgO:Al O :F weight ratio of about l: 1412.2.

5. The process as claimed in claim 3 wherein the phosphoric acid in step (d) is seeded with 1-4 weight percent solid magnesium-aluminum-fluoride-phosphate complex having a MgO:Al O :F weight ratio of about 1 :1 .4122.

6. The process as claimed in claim 3 wherein the phosphoric acid in step (a) is evaporated to a P 0 content of 4751 weight percent P 0 7. The process as claimed in claim 6 wherein the phosphoric acid in step (a) is evaporated until the F/Si0 weight ratio is between about 10 and 20.

8. The process as claimed in claim 7 wherein the phosphoric acid solution in step (a) is seeded with l-4 weight percent solid magnesium-aluminum-fluoride-phosphate complex having a MgO:Al O :F weight ratio of about l:l.4:2.2.

9 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 2,439 Dated February 15,1972

lnvntofls) William P. Moore et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 57, "50l00bLC." should read 50l00C.-.

Column 3, line 60, "50l00bLC." should read -50l00C.-.

Signed and sealed this 6th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOI'ISCHALK Attesting Officer Commissioner of Patents 

2. A process for separating magnesium from 25-35 percent wet-process phosphoric acid obtained by treatment of phosphate rock with sulfuric acid and containing dissolved magnesium, silicon, calcium, iron, aluminum, sulfate, fluoride and other impurities, which comprises: a. evaporating the phosphoric acid at a temperature of 85*-100* C. at a pressure below atmospheric to a concentration of 45-53 weight percent P2O5, and until the silicon content of the acid, measured as SiO2, is reduced to less than 0.2 percent, based on the weight of the concentrated acid, and the hydrogen fluoride content is increased; b. maintaining the hydrogen fluoride content of the concentrated phosphoric acid at a F/MgO weight ratio of at least 2.2; c. maintaining the soluble aluminum content of the concentrated phosphoric acid, measured as Al2O3, at an Al2O3/MgO weight ratio of at least 1.4; d. maintaining the concentrated phosphoric acid at 50*-100* C. for 15-40 hours to form a precipitate comprising a magnesium-aluminum-fluoride-phosphate complex compound; and e. separating the precipitate from the purified concentrated phosphoric acid.
 3. The process as claimed in claim 2 wherein the F/MgO weight ratio in step (b) is maintained between about 3 and 12 and the Al2O3/MgO weight ratio in step (c) is maintained between about 3 and
 12. 4. The process as claimed in claim 3 wherein the phosphoric acid in step (a) is seeded with 1-4 weight percent solid magnesium-aluminum-fluoride-phosphate complex having a MgO: Al2O3:F weight ratio of about 1:1.4:2.2.
 5. The process as claimed in claim 3 wherein the phosphoric acid in step (d) is seeded with 1-4 weight percent solid magnesium-aluminum-fluoride-phosphate complex having a MgO:Al2O3:F weight ratio of about 1:1.4:2.2.
 6. The process as claimed in claim 3 wherein the phosphoric acid in step (a) is evaporated to a P2O5 content of 47-51 weight percent P2O5.
 7. The process as claimed in claim 6 wherein the phosphoric acid in step (a) is evaporated until the F/SiO2 weight ratio is between about 10 and
 20. 8. The process as claimed in claim 7 wherein the phosphoric acid solution in step (a) is seeded with 1-4 weight percent solid magnesium-aluminum-fluoride-phosphate complex having a MgO:Al2O3: F weight ratio of about 1:1.4:2.2. 